1. Field of the Invention
This invention relates to a scanning optical system in which a ghost image is eliminated.
2. Description of the Prior Art
In recent years, a technique of scanning the surface or a photosensitive medium with a modulated light beam to effect recording has often been used. It is known that when a surface to be scanned is scanned by a deflector using a plurality of reflecting surfaces such as a polygon mirror, the scattered light beam created on the surface to be scanned can become a ghost image and imparts an undesirable influence to the image. Means for eliminating such ghost image are disclosed in U.S. Pat. No. 4,040,737. The technique thereof is shown in FIG. 1 of the accompanying drawings. If a scanning optical system is used, in which a parallel light beam Lc impinges on the reflecting surface 3a of a deflector 3 and the deflected light beam Ld is imaged on a medium 6 to be scanned by a rotation-symmetric optical system 7 to cause the light beam Lc to further impinge at an angle relative to a plane perpendicular to the rotational axis 8 of the deflector 3, elimination of the ghost becomes possible. That is, by a slit 9 being disposed between the optical system 7 and the medium 6 to be scanned and adjacent to the medium 6 to be scanned, the ghost image Pg formed in the direction orthogonal to a scanning line 10 can be intercepted.
In the field of such a scanning optical system, there is known a system which prevents the position of the scanning line on the surface to be scanned from being varied by the falling of the deflecting-reflecting surface of the deflector or the falling of the rotational axis of the deflector. FIG. 2 of the accompanying drawings shows an example of the construction of such scanning optical system. A light beam L emitted from a light source device 1 comprising a light source, a condenser, etc. passes through a linear image forming system 2 such as a cylindrical lens and impinges on a reflecting surface 3a of a deflector 3 comprising a rotatable polygon mirror while being linearly converged. The light beam L is reflected by the reflecting surface 3a, passes through an imaging optical system comprising a spherical single lens 4 and a single lens 5 having a toric surface having a major axis and a minor axis having different refractive powers in two orthogonal directions and impinges on a medium 6 to be scanned, thus forming an imaged spot thereon. This imaged spot scans the medium 6 to be scanned at a predetermined speed with the rotation of the deflector 3.
FIG. 3 of the accompanying drawings shows the optical path in a cross section parallel to the deflecting surface of the above-described construction, in other words, a plane containing the major axis of the single lens 5 and the optical axis of the spherical single lens 4. FIG. 4 of the accompanying drawings shows the optical path in a direction perpendicular to the deflecting plane of deflection of the light beam L as deflected by the deflector 3, and illustrates the influence of the falling of the reflecting surface 3a of the deflector 3. The light beam L emitted from the light source device 1 is linearly imaged near the reflecting surface 3a of the deflector 3 by the linear image forming system 2. The refractive power of the single lens 5 in the cross section of FIG. 4 differs from the refractive power of the single lens 5 in the deflecting plane of FIG. 3, and in the imaging optical system comprising the spherical single lens 4 and the single lens 5, the positional relation between the reflecting surface 3a of the deflector 3 and the medium 6 to be scanned is an optically conjugate relation. Accordingly, even if the reflecting surface 3a is inclined from a direction perpendicular to the deflecting plane during rotation of the deflector 3 and changes to a position 3A, the light beam L passing through the imaging optical system comprising the single lenses 4 and 5 changes as indicated by dotted lines but yet no change of the imaged position thereof on the medium 6 to be scanned occurs.
Again in such a scanning optical system, as shown in FIG. 5 of the accompanying drawings, the light beam L having impinged on a point Ps on the medium 6 to be scanned is diffusion-reflected on the surface of the medium 6 to be scanned, and the reflected light La thereof passes through the single lenses 5 and 4 and again impinges on the deflector 3, as indicated by dotted lines. At this time, the reflected light La from the medium 6 to be scanned which has impinged on the reflecting surface 3a is reflected toward the light source device 1 side, while part of the reflected light La from the medium 6 to be scanned impinges on a reflecting surface 3b adjacent to the reflecting surface 3a and is again reflected and passes through the single lenses 4 and 5. That light beam Lb concentrates in the vicinity of the point Pg on the medium 6 to be scanned. This light beam Lb becomes a ghost image and, if a photosensitive medium is installed on the medium 6 to be scanned, there will be formed an undesirable image.
In such a scanning optical system wherein the falling of the deflecting-reflecting surface is corrected, the linear image near the deflecting-reflecting surface 3a and the point on the surface of the medium 6 to be scanned are in a conjugate relation as shown in FIG. 4 and therefore, even if the incident light beam L is inclined relative to the rotational axis of the deflector as shown in FIG. 5, there is a problem that a ghost image is formed on the same scanning line.